Thin-film semiconductor devices such as thin-film transistors (TFTs) are conventionally used in active-matrix display devices such as liquid crystal display devices, as well as in solid state imaging devices such as digital cameras. In display devices, a TFT is used, for example, as a switching element for selecting a pixel, a driver transistor for driving a pixel, or as a driver external to the display region.
For example, an organic electroluminescent (EL) display including organic EL elements utilizing an organic light-emitting material differs from a voltage-driven liquid crystal display in that it is a current-driven display device. Therefore, development of a better performing TFT is rapidly progressing.
Recent years have seen active development of a TFT using an oxide semiconductor, a representative example of which is IGZO (In—Ga—Zn—O) (for example, see Patent Literature (PI) 1 and 2). Structures of TFTs using an oxide semiconductor include a bottom-gate structure and a top-gate structure. Top-gate oxide semiconductor TFTs are capable of reducing parasitic capacitance between the gate electrode and the source and drain electrodes, making them high-performing TFTs.